Bright, noniridescent structural coloration from clay mineral nanosheet suspensions

Structural colors originate by constructive interference following reflection and scattering of light from nanostructures with periodicity comparable to visible light wavelengths. Bright and noniridescent structural colorations are highly desirable. Here, we demonstrate that bright noniridescence structural coloration can be easily and rapidly achieved from suspended two-dimensional nanosheets of a clay mineral. We show that brightness is enormously improved by using double clay nanosheets, thus optimizing the clay refractive index that otherwise hampers structural coloration from such systems. Intralayer distances, and thus the structural colors, can be precisely and reproducibly controlled by clay concentration and ionic strength independently, and noniridescence is readily and effortlessly obtained in this system. Embedding such clay-designed nanosheets in recyclable solid matrices could provide tunable vivid coloration and mechanical strength and stability at the same time, thus opening a previously unknown venue for sustainable structural coloration.


Fig. S1
: Na-Fluorohectorite structure. The orange octahedral sites (pink sphere) contain magnesium partially substituted by lithium. The octahedral sheet is sandwiched in between the blue tetrahedral sheets. The tetrahedral sites (dark blue spheres) contain silicon. The light blue spheres are fluorine, and the red spheres are oxygen. The green spheres are the interlayer cations, typically Na + from the synthesis.  . The PXRD pattern of the ordered interstratification of Na + and Cs + interlayers. The low (0.7%) coefficient of variation indicating the high degree of order in the alternating structure. The 001 peak is located in 2.3 nm, been a sum of Na + cation with interlayer water (1.2 nm) and Cs + interlayer which is not solvated with water (1.1 nm) see insert.

A) B)
Small Angle X-ray Scattering (SAXS) of DBL suspensions were recorded for a series of suspensions with d-spacings giving peaks inside the instruments accessible q-range., see Fig. S5. We observed a series of 00l-reflections indicating the formation of excellent 1D crystalline order of Bragg stacks in the DBL suspension. With increasing concentration, the peak positions shift to high q-values, indicating a reduction of separation between the adjacent DBL nanosheets. The first minimum of the form factor oscillation was observed at 2.1 ± 0.1 nm corresponding to sum of 2 SGL of 0.85 nm and an interlayer height of 0.4. Independent of clay concentration, all scattering curves showed this same minimum, see Fig. S5A, which confirms that all nematic suspensions contain same DBL nanosheets separated to different distances. In principle, the effective refractive index could be determined using SAXS and RSP data in the Bragg-Snell law, but the broad SAXS peaks (Fig. S5B) with corresponding large uncertainties in the estimate of d makes this impossible. Fig. S5C is merging of Fig S5A and B for the most concentrated sample (7.21%).

A) B) C)
Birefringence (BF) measurements were performed in sequence immediately after RSP characterization. In Fig. S6 we show the full range of structural color and corresponding BF for the first and second order colors. All BF images show a vertical narrow feature that comes from the syringe needle sample insertion. The first order presented more uniform colors of DBL in suspension than second order color. With decreasing concentration, the second-order colors, DBLs have more translational and rotational freedom giving less orientational order and producing asymmetrical structural colors. This reduction could render non-uniform distribution of DBLs in suspension yielding regions with different d-spacings and thus heterogeneous structural colors. This is also evident for the RSPs in the R2 range, where the spectra for each sample have broader spectral maxima, and in some cases more than one peak in the same sample. The spectral maxima positions were determined by fitting a second-degree polynomial both for first and second order colors. First, the spectra are normalized, and then the peak wavelength was chosen at the polynomial spectral maxima position, see Fig. S7. Figure S7: Spectrum of 1.40 vol % sample. The fit was done using 2 nd degree polynomial.
Samples began to exhibit some iridescence s after some resting time. This is shown in Fig. S8 below.     Video S1.

Visualization of three examples structural colors in quartz cuvettes on top of a dark background with translational movement.
Video S2.
Visualization of the change from the black background to white background under the structural colors in quartz cuvette.

Video S3.
Tunability of strutural color by adding water in the suspenstion. In half of the quartz cuvette is inserted a clay double layer suspension with a light blue color. The other half is filled with water. After mixed with the syringe needle, the sample presented a wide range of structural color. This diversity of structural colors are a result of the incomplete homogenization of the suspension, which resulted in regions with different concentrations across the sample inside the cuvette resulting in a broad range of structural colors.